This is the cube shaped connector for the round pipes.

There are two types, "2 direction type" and "3 direction type".　It can handle various pipe size. Customization is done by changing the parameters of fusion360 file (.f3d).
Try this connector once.

[Features]
-Customizable
-Fixed firmly
-Hard to break even if squeezed strongly with a wrench
-Easy to remove from the pipe

[Design point]
-Designed so that when the bolt is tightened, the bolt is parallel to the pipe. This enabled stable tightening.

[Bolt/Nut type to use]
type: Countersunk screw bolt and Nut M3 - M6 (If the screw heads do not hit each other, you can use screws of other shapes.)
bolt length: approximately equal to or longer than the pipe diameter.

[Customizable parameters]
-pipe inner diameter (min:8mm at M3)
-pipe outer diameter
-screw size (M3, M4 ,M5, M6)

Detailed screw dimensions are set automatically.
Screws larger than M6 are also possible by manually setting the screw related parameters.

[print conditions]
support: yes
brim: no
filament: PETG, may be PLA, ABS

[note]
-Inner is common to 2 directions and 3 directions.

[caution]
-If you get an error while changing the parameters, click the OK button in the parameter change window, and then click the undo button to return. This is because even if you try to change the parameters without undo to eliminate errors, the abnormal state may be maintained.

[revision]
2021/04/13(v4)
Changed the default cut angle value from 45 degrees to "90-acos (sqrt (2/3))" degrees.

2021/04/10(v3)
I reviewed the design a bit to allow unsupported printing by setting the cut angle to a specific value "90-acos (sqrt (2/3))". The default cut angle value remains at 45 degrees.

2021/04/08(v2)
-Changed the cut angle from 30 degrees to 45 degrees. The angle used for the cap is 30 degrees, but I thought that the connector needed a little more strength, so I changed it to 45 degrees.
-The hole diameter for bolts has been increased a little.

Since the default value of the cut angle is set to "90-acos (sqrt (2/3)) = 54.73561degrees", it is possible to print in the posture shown in the figure below. This allows printing without support. Further, there is an advantage that the strengths in the three directions are the same.
However, the arm will be a little longer, so if you need a shorter arm, you should consider setting the cut angle to 45 degrees or 30 degrees.